Lab 4 - External and Internal Anatomy of the Spinal Cord

Part II - Somatosensory System

Sensory systems are defined as afferent pathways that ultimately reach the cerebral cortex and give rise to conscious perceptions when stimulated. That is, a sensory pathway consists of a chain of neurons originating in peripherally located structures, e.g., in the posterior root ganglia or eye, and terminating in the cerebral cortex. A typical sensory pathway often includes “relay” neurons in the spinal cord and/or brain stem and in the thalamus. The thalamus serves as a “gateway” to the cerebral cortex and conveys sensory information to specific cortical areas called primary cortical receiving areas. Afferent pathways that carry information that does not reach consciousness, such as the posterior spinocerebellar tract, are not considered to be sensory pathways.

In this Exercise, we focus on one of the afferent pathways of the somatosensory system: the dorsal column-medial lemniscal pathway that carries information about discriminative touch and proprioception from the body. Note that this pathway is named after its major ascending tracts, the dorsal column and medial lemniscus. This somatosensory pathway processes the conscious “body” sensations of touch and limb position/movement. It is a rapidly conducting, precisely topographically organized pathway that processes information used in fine tactile discrimination and proprioception (the sense of limb/digit position and movement). You will first study two types of cutaneous receptors, the Meissner corpuscle and the Pacinian corpuscle, which serve as somatosensory receptors of the dorsal column-medial lemniscal system. These encapsulated receptor endings are sensitive to different aspects of cutaneous stimuli: the Meissner corpuscle to discrete (small) tactile stimuli and the Pacinian corpuscle to time varying (vibrating) stimuli. You will also be following the course of tactile and proprioceptive information from the skin, joints and muscles along the dorsal column-medial lemniscal pathway, from the first-order (1°) afferents that form the somatosensory receptors up to and including the ventral posterolateral (VPL) thalamic nucleus. You have already identified the primary cortical receiving area of the dorsal column medial lemniscal pathway, the postcentral gyrus of the parietal lobe, in earlier exercises. This area receives axon terminals from the thalamus.

At the end of this part of the exercise you should be able to:

  1. Identify two types of dorsal column somatosensory receptors and describe their functions.
  2. Describe the course of the dorsal column somatosensory pathway from the receptors to the thalamus.
  3. Identify the relevant tracts and nuclei.
  4. Identify the location of the cell bodies and synapses of this pathway. Identify where this pathway crosses the midline.
  5. Name the sensory modalities carried by this pathway.